The present disclosure relates to a coupled ring oscillator which is capable of producing a highly accurate fine phase using a plurality of ring oscillators.
To record information on an optical disc medium such as a digital versatile disc (DVD), a Blu-ray Disc (BD), and the like, it is necessary to generate a specific write waveform for reducing interference of a write signal. Generation of such a specific write waveform requires a highly accurate fine phase, and more specifically, a fine phase equal to or finer than one-fortieth of a write data rate is necessary. However, it is difficult to realize such very fine phase accuracy by a single inverter chain (a ring oscillator), because a phase delay thereof is smaller than a delay of a single inverter circuit.
Therefore, conventionally, a finer phase than a phase generable by a single ring oscillator is generated using a plurality of ring oscillators, specifically, by connecting inverter circuits in the plurality of ring oscillators together by phase coupling circuits so that an output phase of each of the ring oscillators is slightly shifted. As another example, a resistance ring in which a plurality of resistors are connected, and connection nodes of the resistors in the resistance ring are connected to connections nodes of phase delay elements in a plurality of ring oscillators, thereby generating a fine phase.
In the above-described coupled ring oscillator including a plurality of ring oscillators and a phase coupling ring comprised of phase coupling circuits each of which couples output phases of inverter circuits between the ring oscillators, the phase coupling ring is electrically disconnected at arbitrary parts to cause the ring oscillators to oscillate in a state where coupling of the output phases of the inverter circuits is weakened between different ring oscillators, and then, the phase coupling ring is closed again, thereby performing initialization.
The prevent inventors have found the following fact. A coupled ring oscillator has the same number of different oscillation states as the number of ring oscillators provided in the coupled ring oscillator. When the coupled ring oscillator falls into an undesired abnormal oscillation state, the oscillation frequency is reduced, and thus, a fine phase cannot be obtained. This mechanism will be hereinafter described.
FIG. 13 is a diagram illustrating a typical configuration of a conventional coupled ring oscillator. The coupled ring oscillator includes three ring oscillators 10 and a single phase coupling ring 20. Each of the ring oscillators 10 includes three inverter circuits 11 which are coupled together to form a ring shape. The phase coupling ring 20 includes the same number of phase coupling circuits 21 as the total number of the inverter circuits 11, i.e., nine phase coupling circuits 21, which are coupled together to form a ring shape. Each of the phase coupling circuits 21 couples an output of one of the inverter circuits 11 in one of the ring oscillators 10 to an output of one of the inverter circuits 11 in another one of the ring oscillators 10 in phase or in reverse phase with respect to one another. Then, outputs P1-P9 of the ring oscillators 10 serve as outputs of the coupled ring oscillator. Each ring oscillator 10 can generate only the same number of phases as the number of the inverter circuits 11 provided in the ring oscillator 10 (e.g., three in the example of FIG. 13). However, when being combined, a plurality of the ring oscillators 10 can generate more phases (e.g., nine in the example of FIG. 13). For convenience, it is assumed that each of the phase coupling circuits 21 is an inverter circuit, the following description will be given.
When a signal passes through one of the phase coupling circuits 21, a phase is delayed by an angle obtained by adding θ which is an internal delay of the phase coupling circuit 21 (e.g., a time required for an electron passes through the inside of a transistor, and the like) to 180° caused by logic inversion. That is, a phase difference φ (which will be hereinafter referred to as a “phase coupling angle”) between two points coupled by a single one of the phase coupling circuits 21 is expressed by:φ=180+θ
In the coupled ring oscillator, adjacent outputs (e.g., the outputs P1 and P2) are coupled via two of the phase coupling circuits 21, and thus, a phase difference between the adjacent outputs is 2×(180+θ). Since a phase difference of 360° can be considered as 0°, the phase difference between the adjacent outputs is 2θ. Considering the symmetry of the circuit, a phase difference between any adjacent two of the outputs is 2θ, which is a minimum phase difference that the coupled ring oscillator can generate. When the coupled ring oscillator is in a stable state, a phase of each output is returned to its original phase with one cycle delay (i.e., 360°) due to the periodicity of the phase. Therefore, the minimum phase difference 2θ of the coupled ring oscillator of FIG. 13 is 40°, which is one ninth of 360°. Assuming that p is the number of the inverter circuits 11 in each of the ring oscillators 10, and q is the number of the ring oscillators 10, when θ is generalized, θ can be expressed by following equation:θ=180/pq 
The smaller the internal delay θ of the phase coupling circuit 21 is, i.e., the smaller the phase coupling angle φ is, the more stable state of the coupled ring oscillator can be obtained. When the phase coupling circuit 21 is an inverter circuit, the minimum value of the phase coupling angle φ is 180°. When each of the ring oscillators is configured to include four stages of single-ended inverter circuits, the phase coupling angle φ is maximum, i.e., 270°. Therefore, it is presumed that, when 180<φ≦270°, the coupled ring oscillator stably oscillates. It is also presumed that, when the phase coupling angle φ is out of the range indicated above, the coupled ring oscillator falls into an abnormal oscillation state.
Next, a general expression for the phase coupling angle φ is derived. For convenience, each of p and q is an odd number. First, the minimum value φmin of the phase coupling angle φ is expressed by the following equation:φmin=180+180/pq The phase difference between the outputs of two of the inverter circuits 11 connected together in each of the ring oscillators 10 (e.g., the outputs P1 and P7 in the example of FIG. 13) is q times of the phase coupling angle φ (i.e., qφ), since a signal passes through q phase coupling circuits 21 in the phase coupling ring 20. Then, based on the above,
                              q          ⁢                                          ⁢          φ                =                              q            ⁢                                                  ⁢                          φ              min                                +                      360            ⁢            n                                                  =                              q            ⁡                          (                              180                +                                  180                  /                  pq                                            )                                +                      360            ⁢            n                                                  =                  180          ⁢                      (                          q              +                              1                /                p                            +                              2                ⁢                n                                      )                              Note that n is an integer of 0 or greater, and 360n is a phase delay determined in consideration of the periodicity of the phase. Therefore, the phase coupling angle φ is expressed by the following general expression:φ=180(pq+1+2np)/pq 
Furthermore, when m=(pq+1+2np)/2, the above-indicated general expression can be changed to:φ=360m/pq This shows that the phase coupling angle φ corresponds to the output phase of one of the inverter circuits 11 in one of the ring oscillators 10. That is, the phase coupling circuit 21 might couple the outputs of two of the inverter circuits 11 together with an unintended phase coupling angle φ, and thus, the oscillation operation of the coupled ring oscillator might be stabilized in the unintended phase coupled state.
In the coupled ring oscillator of FIG. 13, a phase coupling angle φ relative to n and a phase difference Δφ between the inverter circuit 11 and the phase coupling circuit 21 are as follows. Note that, assuming that a phase difference between input and output of the inverter circuit 11 is θ′, Δφ is defined as Δφ=360−(θ′+φ). As shown in FIG. 13, in the ring oscillator 10 including three inverter circuits 11 coupled together to form a ring shape, θ′=120.
nφΔφ0200401320−802440 (=80)1603560 (=200)404680 (=320)−80Here, when n≧3, the same state is held as a state where there is a remainder when n is divided by 3. Based on this, it is understood that the coupled ring oscillator of FIG. 13 has three coupled states or oscillation states. The phase difference Δφ is 40°, when n=0. Also, the phase difference Δφ is −80° when n=1, and is 160° when n=2. That is, the phase difference Δφ greatly differs according to the state of the coupled ring oscillator. When an absolute value of the phase difference Δφ is large, a logic collision between the H level and the L level occurs between the ring oscillator 10 and the phase coupling ring 20, and a through current flows, so that the slew rate of an output signal is reduced. As a result, the oscillation frequency of the coupled ring oscillator is reduced. Therefore, to generate a fine phase by the coupled ring oscillator, in other words, to cause the coupled ring oscillator to oscillate at a high frequency, the absolute value of the phase difference Δφ is preferably set to be as small as possible.
In general, a coupled ring oscillator including q ring oscillators has q oscillation states. Normally, the phase coupling angle φ is in the range from 180° to 270°, and thus, an abnormal oscillation state where n is 1 or larger is hardly caused. However, when, in order to obtain a finer phase, the number of inverter circuits provided in each ring oscillator is increased, or the number of ring oscillators provided in the coupled ring oscillator is increased, the number of unintended phase coupling states is increased accordingly, and thus, the risk that the coupled ring oscillator may fall into an abnormal oscillation state is increased. Specifically, when a great number of ring oscillators are provided, or when each of inverter circuits of the coupled ring oscillator has low drive capability, the phase coupling strength between the ring oscillators is reduced, and more phase errors are accumulated, so that the coupled ring oscillator possibly falls into an abnormal oscillation state. Once the coupled ring oscillator falls into an abnormal oscillation state, the coupled ring oscillator is stabilized in the abnormal oscillation state, and thus, the coupled ring oscillator cannot be recovered to a normal oscillation state. This problem cannot be solved by a conventional initialization method in which the phase coupling ring is electrically disconnected at arbitrary parts.